Superconducting oxide materials with a high superconducting transition temperature (Tc), such as LiTi2O3, Ba(Bi, Pb)O3, (Ba, K)BiO3, (La, Sr)2CuO4, REBa2Cu3O7−δ (RE is a rare earth element), Bi2Sr2Ca2Cu3O10, Ti2Ba2Ca2Cu3O10, or HgBa2Ca2Cu3O8, have been discovered one after another in recent years. Superconductors composed of these materials are able to generate a powerful electromagnetic force through interaction with a magnetic field, and their practical application in various fields in which this force is utilized, such as bearings, flywheels and load transport system has therefore been studied.
Of these superconducting oxide materials, those based on REBa2Cu3O7−δ in particular (hereinafter referred to as “RE123 oxide superconducting materials”; the RE here is one or more members of the group consisting of Y, La, Pr, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, and Lu) have a high critical temperature. In addition, they have a high critical current density in a magnetic field due to development and improvement in their manufacturing technology and recently have become one of the most noteworthy superconducting materials.
It has also become clear that superconductors with such a large critical current can function as permanent magnets by trapping a strong magnetic field or, conversely, can shield a strong magnetic field, so in addition to the applications mentioned above, applications such as magnetic shields and permanent magnets are also on the horizon.
The most common method to produce an oxide superconductor (bulk) is a “melt solidification process,” in which a molten oxide superconducting material (crystal precursor) is solidified while being slowly cooled from near its solidification temperature, the result being the growth of crystals. Another manufacturing method is the “supercooling melt solidification process,” in which the crystal growth time is shortened. This method involves supercooling a molten crystal precursor down to a temperature below the solidification temperature while the precursor is still in a molten or semimolten state, then slowly cooling from this temperature or maintaining this temperature to grow crystals. The goal here is to raise the crystal growth rate through supercooling (Japanese Patent Publication H6-211588).
Still, the surface area of a superconducting oxide material needs to be increased if the superconductor is to be used as a magnetic shield or other such material as mentioned above. Furthermore, oxide superconductors have anisotropy in materials properties depending on their crystal orientation, with the current flowing mainly in the a-b direction of the crystals, so for them to be used as a magnetic shield, the sample should be installed so that the c axis is perpendicular to the magnetic field.
However, at the present time the bulk superconductors obtained by the above method are only a few centimeters at the largest size, and it is extremely difficult to produce larger superconductors.                This means that a large superconductor must be produced by joining small superconductor blocks.        
Some of the joining methods known in the past are introduced below.
(1) K. Salama and V. Selvamanickem (Appl. Phys. Lett. 60 (1992), 898)                Samples are joined, without any deterioration of superconducting properties at the joined interface, by heating YBa2Cu3O7−δ superconductors (Y123), which were produced by melt process, for approximately 30 hours under uniaxial pressure of 2 to 6 MPa at a temperature between 910 and 930° C., without interposing solder material between the joined interfaces.        
(2) “Advances in Superconductivity VII,” Springer-Verlag, Tokyo, 1995, pp. 681–684                A matrix of a Y123 superconducting bulk (Y1.8Ba2.4Cu3.4Oy) is prepared. A powder solder of a Yb123 superconducting material (Y1.2Ba2.1Cu3.1Oy) produced by melt solidification process and having a lower melting point (peritectic point) than the matrix is sandwiched between said matrices. The temperature is raised to between the melting point of the matrix and that of the solder to bring the solder into a semimolten state. This is then gradually cooled to epitaxially grow crystals of the solder material (Yb123 crystals) from the matrix surface, joining the matrices together via this crystallized solder.        
(3) Japanese Patent Publication H7-82049                A component that will enter the liquid phase at the joining temperature or a component that readily undergoes high-temperature creep, such as a composition based on Ag, BaCuO2—CuO or REBa2Cu3O7−δ (RE=Y, Ho, Er, Tm, Yb) is interposed by coating, vapor deposition, or another such method at the joined interface between matrices of a yttrium-based oxide superconductor produced by the melt solidification process. After that, this sample is heated under pressure for 1 to 10 hours at a temperature of 900 to 990° C. to fuse it, and then the sample is cooled at a rate of 2° C./hour or less to join the matrices together.        
If we use the above joining method (2) as an example of a conventional joining technique, the following problems are encountered.                Specifically, when a solder is sandwiched between matrices and then heated and slowly cooled, the Yb123 crystallization of the semimolten solder gradually proceeds from the Y123 matrix surface toward the center of the solder. So, the final solidified portion of the solder forms unreacted non-superconductive layer at the middle location of the thickness of the solder sandwiched between the matrices.        
The solder (crystal precursor) that has been heated to a high temperature and becomes a semimolten liquid phase includes a non-superconducting BaO—CuO melt and a non-superconducting Yb211 phase. This Yb211 phase reacts with the melt, forming superconducting Yb123 crystals while solidifying, however, a mixture of the above-mentioned non-superconducting portion such as Yb211 phase and BaO—CuO tends to remain in a layer form in the final solidified portion. Also, the solder that has become a semimolten liquid phase contains numerous voids, impurities, and so on, and these also tend to remain as a layer in the final solidified portion.
This is in part due to the so-called “pushing” effect, in which the Yb211 phase, BaO—CuO, impurities, voids and so forth is pushed forward the unsolidified middle portion of the solder. That is caused by the epitaxial growth of Yb123 crystals from the matrix surface toward the middle of the solder during gradual cooling after heating.
FIG. 9 consists of schematic diagrams illustrating this “pushing” effect. FIG. 10 is a schematic diagram illustrating a joint at which pores and segregation products are present as a result of this pushing.
For example, a solder of a Yb123 superconducting material composition is sandwiched between Y123 superconducting bulk matrices and heated until the solder becomes semimolten, as shown in FIG. 9(a). This semimolten solder includes a Yb211 phase, bubbles, and so forth, which are contained in the molten BaO—CuO. As these slowly cool, as shown in FIG. 9b, the Yb211 phase reacts with the BaO—CuO melt, which produces superconducting Yb123 crystals that grow from the Y123 matrix surface toward the center of the solder. The Yb211, bubbles, and so forth present in the unsolidified melt here are pushed away from the growth front of the Yb123 crystal and concentrate in the middle part of the unsolidified solder. As the Yb123 crystals continue to grow and the solder reaches its final solidification stage, as shown in FIG. 9(c), the unreacted Yb211 phase, bubbles, and so forth segregate increasingly towards the center of the solder. The un-reacted Yb211 phase, bubbles, and so forth finally segregate in layer form over the entire cross section of the middle part of the solder, in which state the solidification of the solder is completed, joining the Y123 matrices.
Since the Yb211 phase, bubbles, and so forth are not superconducting, the superconducting characteristics of a joined oxide superconductor by the above method, especially in this joined portion, are markedly degraded.